What this calculator works out
This estimates a due date from the first day of your last period, from a known conception date, or from an IVF transfer. It also shows how many weeks and days pregnant you are today and the window within which a birth is considered term.
The standard method adds 280 days to the first day of the last period, which is why a pregnancy is counted as starting about two weeks before conception actually happened.
It is an estimate, and a broad one. Only around four in a hundred babies arrive on their due date. Your dating scan will give a more accurate figure, and that is the date your midwife will use.
Enter your details
Before you calculate
- The date needed is the first day of your last period, not the day it ended.
- Pregnancy is counted from that day, which is roughly two weeks before conception. It is a convention, not a biological start point, and it is why you are considered four weeks pregnant when the embryo is around two weeks old.
- If your cycle is not 28 days, enter your own figure. A 35-day cycle moves the estimate a week later.
- If you do not know the date of your last period, or your cycles are irregular, a dating scan will give a better answer than any calculation.
Only around four in every hundred babies arrive on their due date. Most are born somewhere between 37 and 42 weeks. The date is best understood as the middle of a five-week window rather than an appointment. Your midwife will confirm a date at your dating scan, and that date replaces this one for the rest of the pregnancy.
How this calculator works
The standard method is Naegele's rule, named after the nineteenth-century German obstetrician Franz Naegele. It adds 280 days — forty weeks — to the first day of the last menstrual period.
Due date = first day of last period + 280 days
Adjusted for cycle = + (cycle length − 28) days
From conception = conception date + 266 days
From day-5 transfer = transfer date + 261 daysThe rule assumes a 28-day cycle with ovulation on day 14. Where a cycle is consistently longer or shorter, ovulation happens correspondingly later or earlier, so the calculator shifts the estimate by the difference. This adjustment is a reasonable correction, not a precise one, because the second half of the cycle is more consistent than the first but not fixed.
The conception and IVF options count forward from a known event rather than backwards from a period, which is why they do not use the cycle length at all. Counting from a day-5 blastocyst transfer uses 261 days because the embryo is already five days old at transfer.
Worked example
A last period beginning on 1 May 2026, with a regular 28-day cycle:
- 1 May 2026 plus 280 days gives a due date of 5 February 2027
- The 37-week point, when a pregnancy is considered term, falls on 15 January 2027
- The 42-week point falls on 19 February 2027
With a 35-day cycle instead, the estimate moves seven days later to 12 February 2027. That is a substantial shift, and it is why the cycle length is worth entering accurately rather than leaving at the default.
Understanding the result
The calculator shows the estimated due date, how many weeks and days pregnant you are today, which trimester that falls in, and the window in which a birth would be considered term.
| Stage | Weeks | Notes |
|---|---|---|
| First trimester | 0 to 13 | Booking appointment usually offered by 10 weeks |
| Second trimester | 14 to 27 | Dating scan and anomaly scan fall around this boundary |
| Third trimester | 28 to birth | Antenatal appointments become more frequent |
| Preterm | Before 37 | Additional care is usually needed |
| Term | 37 to 42 | The normal window for birth |
A pregnancy reaching 40 weeks has been developing for about 38 weeks, because the count starts two weeks before conception. Both statements are correct and they describe the same pregnancy.
Common mistakes
- Using the last day of the period. The calculation counts from the first day.
- Leaving the cycle length at 28 when it is not. A week's difference in cycle length moves the estimate by a week.
- Using the last-period method after irregular cycles. If cycles vary substantially, the estimate is unreliable and a scan is the better guide.
- Treating the date as a deadline. Most babies arrive within two weeks either side of it.
- Using a period date after coming off hormonal contraception. The first cycles afterwards are often not representative.
- Assuming the calculated date stands. The dating scan date replaces it, and the scan is more accurate.
Frequently asked questions
How accurate is a due date?
As a single date, not very — only around four in a hundred babies are born on it. As a window, it is useful: most births happen between 37 and 42 weeks, and the great majority within two weeks either side of the estimate. Treat it as the middle of a range rather than a prediction.
Why is pregnancy counted from the last period rather than conception?
Because the date of the last period is usually known and the date of conception usually is not. It is a practical convention that has been used for two centuries. The consequence is that you are counted as around four weeks pregnant when the embryo is about two weeks old.
Will my midwife use this date?
Only until your dating scan. In the UK that scan is normally offered between 8 and 14 weeks, and the date it gives becomes the official due date for the rest of the pregnancy. Scan dating measures the baby directly and is more reliable than counting from a period, particularly where cycles are irregular.
What if my cycle is not 28 days?
Enter your own average length. The calculator shifts the estimate by the difference from 28 days, on the basis that ovulation happens later in a longer cycle. If your cycles vary a lot from month to month, the estimate becomes unreliable and a scan is the better guide.
How does this work for IVF?
IVF dates are known precisely, which makes the estimate more reliable than any period-based calculation. A day-5 blastocyst transfer counts forward 261 days and a day-3 transfer 263 days, because the embryo already has a known age at transfer. Your clinic will give you a date and that is the one to use.
What does term mean?
A pregnancy is considered term from 37 weeks to 42 weeks. Before 37 weeks is preterm and usually needs extra care. After 42 weeks a pregnancy is described as prolonged, and induction is normally discussed well before that point.
Is what I enter stored?
No. Dates are processed in your browser and never transmitted or retained. Nothing about your pregnancy is sent anywhere.
Related tools
References
These organisations publish the guidance and figures behind this page.
- NHS — pregnancy — antenatal care, the dating scan, and what to expect at each stage
- NICE — clinical guidance on antenatal care and pregnancy dating
- Tommy's — pregnancy information and support from the UK pregnancy research charity
This page is general information, not medical advice. Your midwife or GP is the right source for anything concerning your own pregnancy, and a dating scan gives a more accurate due date than any calculation.